[3] File Importer support

[SwiftfulThinking]

Support File Importer (presents a system interface for allowing the user to import multiple files)

https://developer.apple.com/documentation/swiftui/list/fileimporter(ispresented:allowedcontenttypes:allowsmultipleselection:oncompletion:)

// something like...
router.showFileImporter { result in
     // onCompletion
}